Amorphous silicates are substantially weaker when in contact with aqueous electrolyte solutions than in vacuum due to chemical interactions with preexisting fractures. To investigate this effect at the nanoscale, classical molecular dynamics (MD) simulations using the bond-order based reactive force field (ReaxFF) were used to isolate mechanisms of silica fracture. In this investigation, a slit crack was introduced into an atomistic amorphous silica model and mode I stress was applied through far-field loading until the crack propagated. The chemical and mechanical aspects of fracture were separated by control of environmental conditions including: (a) vacuum with dynamic loading, (b) an aqueous environment with dynamic loading, and (c) an aqueous environment with static subcritical mechanical loading to track silica dissolution. The addition of water to dynamic loading reduced the silica fracture toughness (KIC) by ~25%, a trend consistent with experimentally reported results, while water with static loading resulted in almost no crack propagation. Analysis of the impact of aqueous electrolyte solutions included simulations of silica fracture in two salt solutions (1M NaCl, 1M NaOH) that form relatively acidic and basic conditions, respectively. While all aqueous solutions weakened the silica, NaOH additions resulted in the largest decreases in KIC. The electrolyte solutions resulted in two distinct responses in the silica. The basic solution lead to more surface deprotonation, less dissolution, narrower crack tip radii, and greater silica weakening than the more acidic NaCl solution. The divergent results from the two different electrolyte solutions describe phenomena observed in experiments, and provides insight into how anions alter the chemical-mechanical fracture response of silica. The application of the ReaxFF classical MD force field in this study provides the basis for future research into crack growth in multicomponent oxides in a variety of environmental conditions.

Chemo-mechanical fracture of silica in aqueous electrolyte solutions

2019 Spring National Meeting

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